The present invention relates to an internal mirror type laser tube and, more particularly, to a mirror arrangement for a random polarization He-Ne gas laser tube.
In a conventional internal mirror type gas laser tube such as He-Ne gas laser tube, mutually orthogonal polarizing directions of output laser light are arbitrary with respect to an optical axis of the laser light and peculiar to individual laser tubes. This is because relative direction of anisotropic characteristics of a pair of mirrors have never been concerned at all and thus a pair of mirrors constituting an optical resonator have been arranged arbitrarily.
Referring to FIG. 1A, an output mirror 11 and a total reflective mirror 12 have inherent anisotropic characteristics, respectively as shown in dotted ellipse 110 and 120. A means for measuring such anisotropic characteristics will be described later.
When an optical reasonator is constituted by mounting the mirrors 11 and 12 on opposite ends of a laser tube envelope, the resonator exhibits anisotropic characteristics such as shown by a dotted line 100 in FIG. 1B. Mutually orthogonal polarizing directions of longitudinal mode of the output laser light 10 are determined selectively in such directions that oscillation is easier. In other words, internal gain is higher in that directions, or reflection intensity is higher in that directions. Accordingly, if relative directions of anisotropic characteristics of a pair of mirrors are arbitrary, such laser tube has peculiar anisotropic characteristics and polarizing directions.
Considering longitudinal mode oscillation, longitudinal mode oscillation occurs within a peculiar gain curve of an optical resonator as shown by a dotted line 200 in FIG. 2A and adjacent longitudinal modes 201, 202 and 203 have orthogonal polarizing directions ( .multidot..rarw..fwdarw.) and are separated from each other by C/2L, wherein the C represents velocity of light and the L represents resonator length. Further, frequencies of individual longitudinal modes are shifted with even small change of reasonator length due to influence of such as heat.
Assuming that an internal gain of either of mutually orthogonal two polarizing directions, for example, X direction 101 ( direction) is higher than that of the other, Y direction 102 (.rarw..fwdarw. direction), as shown in FIG. 1B, longitudinal mode tends to shift as shown in FIG. 2B to FIG. 2D. When the longitudinal modes are shifted from a state shown in FIG. 2B leftwardly, a right side mode 204 within the gain curve 200 is usually polarized in Y direction. However, since internal gain thereof in Y direction is low and hardly oscillates, it can oscillate in only X direction. Therefore, the stable state in which adjacent longitudinal modes have mutually orthogonal two polarizing directions is broken and, thus, it is shifted to such stable state (FIG. 2C) and, then, to the state shown in FIG. 2D, and the shift from that shown in FIG. 2B through that shown in FIG. 2C to that shown in FIG. 2D is repeated. That is, during this shift process from that shown in FIG. 2B to that shown in FIG. 2C, the polarizing direction of oscillating longitudinal mode is rotated by 90.degree..
At the moment of this 90.degree. rotation, a phenomenon of over rotation tends to occur as shown in FIG. 3. That is, arbitrary arrangement of a pair of mirrors for an optical resonator causes a deformed anisotropic characteristics such as shown in FIG. 3. Although mutually orthogonal longitudinal modes 101 and 102, are easily oscillated in X and Y directions, respectively, directions close to the X and Y directions also have internal gains suitable to oscillate practically. Thus there may be a case where, over rotation occurs as indicated by an arrow 105 and then returns to stable X direction from the over-rotated position 104 as indicated by an arrow 106. This phenomenon causes a problem that an output laser light is abruptly changed due to change of polarizing direction of the laser light in a case, particularly, where laser light is used through optical parts such as polarizer and/or reflectors having anisotropic characteristics.